Re: [Xen-devel] [PATCH 14/18] xen/arm: Unmask the Abort/SError bit in the exception entries

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 22/03/17 08:49, Wei Chen wrote:
> Hi Stefano,
>
> On 2017/3/21 5:38, Stefano Stabellini wrote:
> > On Mon, 13 Mar 2017, Wei Chen wrote:
> > > Currently, we masked the Abort/SError bit in Xen exception entries.
> > > So Xen could not capture any Abort/SError while it's running.
> > > Now, Xen has the ability to handle the Abort/SError, we should unmask
> > > the Abort/SError bit by default to let Xen capture Abort/SError while
> > > it's running.
> > >
> > > But in order to avoid receiving nested asynchronous abort, we don't
> > > unmask Abort/SError bit in hyp_error and trap_data_abort.
> > >
> > > Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
> > > ---
> > > We haven't done this before, so I don't know how can this change
> > > will affect the Xen. If the IRQ and Abort take place at the same
> > > time, how can we handle them?
> > If the abort is for Xen, the hypervisor will crash and that's the end of
> Before the system crash, we have enable the IRQ, so that would not be
> the end if an IRQ happens at the same time.
>
> > it. If the abort is for the guest, Xen will inject it into the VM, then
> Before we have inject the abort to VM, we have enable the IRQ.
>
> > it will return from handling the abort, going back to handling the IRQ
> > as before. Isn't that right?
> If the abort has higher priority then IRQ, it's right.
>
> > > If an abort is taking place while we're handling the IRQ, the program
> > > jump to abort exception, and then enable the IRQ. In this case, what
> > > will happen? So I think I need more discussions from community.
> > Do you know of an example scenario where Xen could have a problem with
> > this?
> For example,
> 1. Trigger a SError in hypervisor.
> 2. Jump to hyp_error to handle SError.
> 3. Enable IRQ in hyp_error before PANIC
> 4. A timer IRQ happens.
> 5. Jump to hyp_irq and unmask abort again.
> 6. Jump hyp_error again?
Technically you could end up in an infinite loop if hyp_error code 
generates a SError. It will stay pending until the end and then trigger 
again when SError is unmasked...
That's unfortunate but I don't think this is a big issue as if this is 
happening your platform is already doomed.
Cheers,

--
Julien Grall

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